When a head in a disk drive is moved by its corresponding servo-controlled actuator from one disk track to another selected disk track, a servo system controls the actuator and closely monitors and controls the velocity and position of the head on the disk in real time to achieve optimal performance. For this purpose, as is well-known to one of ordinary skill in the field, position information is derived and fed back in the form of an error correction signal to a compensator circuit. The compensator circuit generates a signal that is applied to the actuator to accurately control the radial position of the head over the disk.
The operating phase margin of the servo control circuit is typically preset for large numbers of disk drives based on an assumed set of worst case conditions. However, past experience has shown that actuator friction varies from drive to drive over a large population of drives and that friction also varies with changes in operating temperature. It is well-known that as friction varies, so does the phase margin of the servo system, which directly affects the stability and performance of the disk drive over time.
Of course, when the phase margin for the servo control is set to an assumed worst case situation, individual disk drives not operating in the worst case do not receive optimal servo control information. Further, when actuator friction changes under normal operating temperature changes the preset phase margin compensation does not change which results in the inability of the servo control to optimize to actual operating conditions for a specific disk drive.
Previously known control systems for optimizing the seek performance of disk drives in real time adjust a seek velocity profile, which controls the velocity at which the actuator is driven as a function of the length of a specific seek to be performed. Such systems monitor the occurrence of head overshoots and related parameters to update the seek velocity profile. These systems do not account for the reduction in performance caused by a suboptimal phase margin.